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Pheromones, airborne chemical signals released by humans from skin glands into the environment that have an affect on the physiology or behavior of other members of the same species, were the catalyst for the synchronization of ovulatory cycles. The initial hypothesis was that roommates’ cycles would have the highest correlation, because they had the strongest shared air space. However, the studies showed that good friends had the closest timing, after 3 to 4 months together.

Given that pheromones are sent out by women at different phases in their ovulatory cycle, and that most women are receptive (to some degree – variation was noted in receptivity (3)) to those chemicals, the next step is to describe how the pheromone influences the timing of the cycle.

Just as genes do not cause diseases, pheromones do not directly cause behaviors. Jim Kohl (6) explains that pheromones influence gene expression primarily in nerve cells that secrete gonadotropin-releasing hormone (GnRH). GnRH has a regulatory effect on luteinizing hormone and follicular stimulating hormone, both of which are associated with parts of the menstrual cycle. Some of Jim’s supporting evidence was a little iffy – he suggests that pheromones are indicated in coitus-induced ovulations, though I suspect that there may be mechanical explanations of this as well (7). However, the basic mechanism he suggests seems sound. The vomeronasal organ, proteins very similar to smell receptors and in the same nasal region, but encoded by separate genes, detects the pheromones and sends a signal to the hypothalamus. The pheromones are called primer pheromones (10), for their role in priming the endocrine system and to distinguish them from identification hormones that most mammals (think dogs and cats) use to mark their territory. This triggers it to send GnRH to the pituitary gland. The pituitary then sends out corresponding pulses of follicular-stimulating hormone and luteinizing, which correspond to the production of estrogen and progesterone in females, which have direct bearing on the menstrual cycle.